Recent advances in optics and electronics have culminated in the development of the laser confocal scanning microscope (LCSM). The LCSM provides unparalleled resolution for localizing fluorescent probes in cells. In combination with computers, it is relatively easy to produce 3-dimensional reconstructions of fluorescently labelled structures in cells. In the past year we installed a state of the art BioRad MRC 600 LCSM with its controlling computer. We also installed a Silicon Graphic Indigo computer to facilitate manipulation of 3-dimensional data sets generated by the LCSM. We have begun to use this system for a wide variety of applications, including determining the subcellular distribution of proteins, following the intracellular trafficking of newly synthesized membrane proteins, and studying the migration of developing thymocytes in thymic explants.